the weird rules of the subatomic world are very very different to those of the familiar large-scale universe a huge outstanding question is when and why does the weirdness of quantum mechanics give way to classical physics one answer to this question suggests that the entire universe is so much weirder than we imagined or should I say the multiverse one of the strangest features of the quantum description of reality is the idea of superposition we can't describe the most fundamental building blocks of our universe with defined singular properties instead they seem to behave as probability clouds of all properties they might have were we to try to measure them mathematically this is encapsulated in the wave function of a quantum particle or system of particles the best illustration of why we need to describe the quantum world this way is the famous double slit experiments we did an episode on it check it out if you aren't familiar but to summarize a stream of photons or electrons or even molecules travels from some point to a detector screen via a pair of slits these particles arrive at the screen distributed like the interference pattern you expect from a simple wave quantum mechanics very successfully predicts this result by describing each particles journey as a superposition of all possible trajectories in other words the particle simultaneously takes all possible paths which means it passes through both slits it tries out all histories between launch and landing and those many maybe histories somehow interact with each other to determine the most likely final destination when a measurement is made in a sense different possible superposed histories appear to converge on one final outcome but what causes that convergence in the original Copenhagen interpretation of quantum mechanics the act of measurement was thought to collapse possibility space into a single reality at least with respect to the measured property it collapses the wavefunction that collapse signifies the transition between the quantum and classical realms one of the founders of quantum mechanics Erwin Schrodinger found this ridiculous and he proposed his famous Schrodinger's cat thought experiment to highlight the absurdity it goes like this and cat is in a box with a classical poison a machine containing a radioactive element is set to shatter the flask in the event that the radioactive element decays if that happens the cat dies that radioactive decay is a Hughley quantum process and so until it's observed it exists in a superposition of states it has both decayed and not decayed but doesn't that mean that the entire macroscopic system attached to that quantum event is also in superposition if so then the cat should be simultaneously alive and dead until we open the box but why can't the cat collapse its own wavefunction and from its point of view is the physicist outside also a quantum blur until the box is opened and what about the entire rest of the universe that's not currently being observed by physicists or cats many adherents to Copenhagen now have a more sensible resolution to the paradox of Schrodinger's cat it's that quantum superposition doesn't extend to macroscopic scales it disappears when different quantum scale histories diverge this is called decoherence when the wave functions describing quantum systems overlap sufficiently in other words they are coherent it's possible to get interference in the double slit experiment and spookley correlated quantum entanglement measurements but when these systems interact with their environment coherence is lost and parallel history is fall out of alignment they can no longer interact with each other by the Copenhagen interpretation we might say that the universe chooses the final outcome of all of those histories it doesn't exactly choose history instead it chooses an end result say particle location on a screen or cat alive or dead nurse based on those histories if a larger number of possible histories lead to a given result then it's more likely that the universe will select that outcome the Copenhagen interpretation says that this selection happens in a fundamentally random way the universe players dice even if the dice are weighted towards certain results it is what we would call a non-deterministic interpretation because there's no underlying predictability behind the selection however there is another way to interpret the transition between the quantum and classical worlds what if the wavefunction never collapses if we can imagine a cat in a superposition of states alive and dead why stop at the cat what if the family of possible states extends beyond the radioactive decay beyond the cat and includes the observer and indeed the entire universe - if we open the box and find that the cat is alive it's because we are part of an entire quantum timeline in which the radioactive decay and subsequent poisoning never happened but there's an equally valid timeline in which it did and another version of us experiencing that this sounds outrageous but it's a very serious interpretation of the mathematics of quantum mechanics it was proposed by Hugh Everett in his 1957 PhD thesis entitled the theory of the universal wavefunction it's come to be known as the many-worlds interpretation to outline the idea without killing so many cats let's talk about what this means in the context of the double slit experiments the Copenhagen interpretation tells us that the superposition of particle trajectories of histories merges into the single timeline of the observers reality many-worlds says this merging never happens those alternative histories continue and we find ourselves in just one of those timelines which one well they're all equally likely but some look very similar to each other for example many histories lead to photons landing on the bright bands of the interference pattern and very few to the dark bands we tend to find ourselves in the more common families of histories this is a pretty crazy notion the many-worlds interpretation invites the idea that reality splits into different branches every time quantum states diverged into different possibilities for example at every particle interaction everywhere in the universe this would lead to an unthinkably large number of alternate timelines or worlds that contain all possible realizations of this universe since the Big Bang it seems extravagant to propose uncountable eternally branching universes just to get out of collapsing a wavefunction it's like building an entirely new house to avoid doing the dishes but remember the Copenhagen interpretation itself proposes multiple worlds in the superposition of paths or properties of a quantum system both many worlds and Copenhagen create alternate realities it's just that Copenhagen merges them into a single timeline with its wavefunction collapse the superposition of states of many worlds can be thought of as overlaid histories slices of a universal wavefunction that diverge from each other as the universe evolves but none ever vanish many worlds may in fact be the more pure interpretation of the mathematics of quantum mechanics because the nothing in that math that requires the collapse of the wavefunction so many worlds is more economical in the number of unsupported concepts it adds to quantum mechanics even if it isn't particularly economical in the number of universes it predicts now Everett's idea wasn't taken too seriously when it was first proposed that may have been in part because he wasn't a well-known physicist he was just a graduate student who all but disappeared into military research at the Pentagon right after graduation but another point of resistance must be the overwhelming existential crisis induced by the idea of near infinite versions of oneself many worlds may imply that every possible version of you exists out there you're just the one who happens to be experiencing this branch of reality every other possible life path including those branching in different directions from every decision you ever made may be just as real in fact each may be real in vast multitudes there's no more evidence for many worlds than there is for other mainstream interpretations of quantum mechanics and it is somewhat mainstream these days with many noted physicists being swayed by its parsimony its economy of ideas but it remains an interpretation and so although it is supported by the incredibly successful mathematics of quantum mechanics it is not yet added a prediction that might distinguish it from other equally supported interpretations nor is it complete in its explanation there are some ideas about what's really happening when these neighboring coherent histories interact or why the wavefunction translates to probabilities the way it does but they are far from generally accepted unlike Copenhagen many worlds is a deterministic interpretation any given time line is a predictable chain of cause and effect it explains the apparent randomness of quantum mechanics with a sort of observer bias all possibilities are chosen at every Junction and we just happen to be seeing the one happened in the branch that we occupy that adds a second possible cause for philosophical and ears in a purely deterministic universe what happens to freewill if we're going to make all possible decisions anyway why sweat any given choice or sure but of all those countless future branches of reality some are going to be pretty amazing think of it as a choose-your-own-adventure and steer this version of you towards one of the more awesome many world branches of space-time well guys do you know who